Examine source code of Tab Flag

{
  "developer": {
    "name": "EffPeh"
  },
  "description": "This extension flags unread respectively unvisited tabs by prefixing the title with one or more characters/symbols of your choice. So you can see at a glance which web pages have not yet been read and find the corresponding tabs quickly.",
  "manifest_version": 2,
  "name": "Tab Flag",
  "version": "1.2resigned1",
  "icons": {
    "48": "icon.svg"
  },
  "permissions": [
    "<all_urls>",
    "tabs",
    "storage"
  ],
  "options_ui": {
    "browser_style": true,
    "page": "options.html",
    "open_in_tab": false
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "applications": {
    "gecko": {
      "id": "tab-flag@example.com",
      "strict_min_version": "60.0"
    }
  }
}